Cascading Interfaces Enable n-Si Photoanodes for Efficient and

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Cascading Interfaces Enable N-Si Photoanode for Efficient and Stable Solar Water Oxidation Lingyun He, Wu Zhou, Liu Hong, Daixing Wei, Guangxu Wang, Xiaobo Shi, and Shaohua Shen J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00746 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 20, 2019

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The Journal of Physical Chemistry Letters

Cascading Interfaces Enable n-Si Photoanode for Efficient and Stable Solar Water Oxidation Lingyun Hea, Wu Zhoua, Liu Hongb, Daixing Weia, Guangxu Wangb, Xiaobo Shib, Shaohua Shena* aInternational

Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow

in Power Engineering, Xi’an Jiaotong University, Shaanxi 710049, P. R. China bNational

Key Lab of Science and Technology on LRE, Xi’an Aerospace Propulsion Institute, Shaanxi 710100, P. R. China E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: The interfaces with multi-functions for the promoted solid/solid interfacial charge transfer dynamics and the accelerated solid/electrolyte interfacial water redox reaction kinetics are determinative to the photoelectrodes achieving high performances for photoelectrochemical (PEC) water splitting. In this work, the well-designed cascading interfaces are introduced in the n-Si photoanode, which is effectively protected by an atomic layer deposited CoOx thin layer for stabilizing the n-Si photoanode and then coated with an earth-abundant NiCuOx layer for catalyzing water oxidation reaction. Furthermore, the formed n-Si/CoOx/NiCuOx triple-junction could generate a large band bending to provide a considerable photovoltage for promoting the photoinduced charge transfer and separation processes at the n-Si/CoOx/NiCuOx cascading interfaces. Moreover, at the NiCuOx/electrolyte interface, an in-situ electrochemically formed NiCu(OH)x/NiOOH active layer facilitates the water oxidation reaction kinetics. This study demonstrates an alternative approach to stabilize and catalyze n-Si based photoanodes with cascading interfaces for efficient solar water oxidation.


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TOC GRAPHICS

To solve the issues of global warming and environment security caused by fossil fuels, great efforts have been contributed to the alternative and sustainable energy resources.1,2

As

a

promising

approach

to

renewable

energy

conversion,

photoelectrochemical (PEC) water splitting could efficiently convert solar energy to chemical energy stored in hydrogen.3 One pivotal challenge toward the realization of applicable PEC systems is to search for the appropriate semiconductor photoelectrodes with suitable band gap well matching the solar spectrum and long-term operation stability in aqueous conditions. Given the narrow band gap for effective absorption of solar light and the high earth abundance, silicon (Si) has been extensively applied in the


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photovoltaics and also exhibited great potential as an attractive photoelectrode material for solar water splitting.4-6 However, because of the intrinsic instability, Si based photoelectrodes are subject to extensive surface corrosion and passivation in aqueous solutions, especially for n-Si photoanodes for water oxidation in alkaline aqueous conditions with high pH values. Furthermore, the sluggish oxygen evolution reaction (OER) kinetics at the n-Si/electrolyte interface, resulting from the Fermi level pinning (FLP) at the n-Si surface, seriously restricts the photoconversion efficiency of n-Si based photoanodes for PEC water oxidation.7,8

To protect n-Si photoanodes from surface corrosion, different methods have been developed to coat n-Si with an overlayer of metals, metals oxides, metal hydroxides and metal phosphides to effectively stabilize n-Si photoanodes.3,9-14 Simultaneously, these overlayers sometimes acting as OER catalysts could significantly boost the OER kinetics, and then the formed n-Si/overlayer heterojunctions could yield a band bending to promote photogenerated charge separation, thus leading to superior PEC performances for water oxidation. Although noble metals (Pt, Ru and Ir) and noble metal


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oxides (RuO2 and IrO2) as the admirable OER catalysts demonstrate these cocatalysts are of great benefit to improve the activity and stability of n-Si photoanodes for water oxidation,15-17 noble metals have great limitations in large-scale application, due to their high cost and scarcity. Therefore, it is highly desiderated to replace noble metal catalysts with earth abundant and low-cost materials for PEC water splitting,9,10,18-28 and some transition metal oxides (such as NiOx, CoOx, FeOx) have been reported to modify n-Si photoanodes with considerable activity and long-term stability over a broad range of pH values.23,24 For example, when the CoOx thin layer was deposited on n-Si photoanode by atomic layer deposition (ALD), the formed n-Si/SiOx/CoOx photoanode exhibited a high photocurrent density (Jph) of 22.9 mA cm-2 (defined as the photocurrent density obtained at 1.23 V vs. reversible hydrogen electrode (RHE)) and a long-term durability (~2500 h) in alkaline solution, ascribed to the formed n-Si/SiOx/CoOx junction providing a photovoltage of 575 mV for efficient charge separation as well as the coated CoOx overlayer catalyzing the water oxidation reaction and protecting n-Si from surface corrosion.27 Cox et al. demonstrated that, with the highly active Co(OH)2 layer spontaneously generated on the Co3O4 protected layer, the constructed p+n-


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Si/Co3O4/Co(OH)2 photoanode could provide a very large photovoltage (~600 mV) to facilitate charge carrier separation and collection for water oxidation, with Jph and saturation photocurrent density (Js) reaching as high as ~30.8 mA cm-2 and ~37.5 mA cm-2, respectively.25 These investigations reveal that the deposited earth-abundant transition metal oxide layers can produce considerable photovoltage at the n-Si/oxide interfaces for efficient charge carrier separation and act as the active layer at the oxide/electrolyte interface for accelerating water oxidation kinetics, in addition to protecting the chemically impressible n-Si from corrosion.22,26,29

As inspired by these researches, herein we demonstrate the well-designed cascading interfaces to the n-Si photoanode with a CoOx thin layer deposited by plasma-enhanced atomic layer deposition (PE-ALD) and then a NiCuOx mixed-valence oxide overlayer coated by spin-coating process to achieve efficient and stable PEC water oxidation. The obtained n-Si/CoOx/NiCuOx photoanode exhibits a low photocurrent-onset potential of 1.04 V vs. RHE (defined as the potential required to achieve an anodic photocurrent density of 1 mA cm-2), a high Jph of 28.3 mA cm-2 and a considerable photovoltage of


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~550 mV for water oxidation. In addition, the composite photoanode operates stably for ~23 h in 1.0 M NaOH under simulated solar illumination without obvious degradation in photocurrent density. The excellent PEC performances should be ascribed to the synergy of the deposited CoOx and NiCuOx layers: 1) the excellent stability owing to the ALD-deposited impact CoOx layer protecting n-Si from surface corrosion; 2) the increased optical absorption with light reflection reduced by the NiCuOx layer; 3) the promoted OER kinetics due to the in-situ generated NiCu(OH)x/NiOOH active layer at the NiCuOx/electrolyte interface; 4) the high photovoltage (~550 mV) produced by the large band bending attributed to the formed n-Si/metal oxides (CoOx/NiCuOx) triplejunction at the n-Si/CoOx/NiCuOx cascading interfaces. This study provides an attractive approach to the design of n-Si based photoanodes, with the well-designed cascading interfaces created by earth-abundant multifunctional metal oxide layers for the highly efficient PEC water oxidation.

In this study, the n-Si/CoOx/NiCuOx photoanodes with the cascading interfaces were prepared by a two-step process, i.e., ALD coating of a CoOx thin layer and spin-coating


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of a NiCuOx overlayer (Figure 1a), with photoanode structure characterized in Figure 1b,c. In the first step, the CoOx thin layer with thickness of ~25 nm (Figure 1c) was coated on the surface of n-Si wafers by ALD (Figure S1a). The CoOx layer is pin-hole free and amorphous, with intact contact formed at the n-Si/CoOx interface (Figure 1d). In the second step, a 95 nm thick and amorphous NiCuOx layer (Figure 1c) was spin coated onto the as-prepared n-Si/CoOx film. The CoOx/NiCuOx double layer is then determined to be totally ~120 nm in thickness, which can be also evidenced by the cross-sectional scanning electron microscopy (SEM) image in Figure S1b. By the scanning transmission electron microscopy (STEM) element mapping and the corresponding line scan compositional distribution depending on the depth of the nSi/CoOx/NiCuOx photoanode (Figure 1e and Figure S1c), On can easily observe that the region of element nickel is almost identical to that of the element copper, and the cobalt region is below the nickel region, demonstrating the well-defined double layers of CoOx and NiCuOx successively deposited on n-Si wafers via the two-step process.


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Figure 1. Structure characterizations of the n-Si/CoOx/NiCuOx photoanode: a) The schematic of the n-Si/CoOx/NiCuOx photoanode fabricated by an ALD and spincoating

two-step

processes;

b)

Cross-sectional

TEM

image

of

the

n-

Si/CoOx/NiCuOx photoanode; High-resolution TEM images of c) n-Si/CoOx/NiCuOx structure and d) the n-Si/CoOx interface; e) STEM-energy dispersive spectroscopy (STEM-EDS) line mapping and element mapping of n-Si/CoOx/NiCuOx The PEC water oxidation performances of these photoanodes were measured by photoanode, Si was shown in yellow, O in cyan, Co in magenta, Ni in green and cyclic voltammetry (CV) scan in a three-electrode system in 1.0 M NaOH solution under Cu in red. simulated solar illumination. The n-Si/CoOx photoanode obtained in the first ALD step shows a reasonable PEC performance for water oxidation (Figure S2a), with Jph reaching 4.9 mA cm-2 and photocurrent-onset potential at 1.14 V vs. RHE. For


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comparison, polished bare n-type (100) Si was also tested for PEC water oxidation (Figure S2a, inset). Additionally, the n-Si/CoOx photoanode presents a high Js reaching 26.5 mA cm-2, which is close to the upper limit of the theoretical value. It was evidenced that the CoOx layer deposited on n-Si wafer would introduce an n-Si/CoOx Schottky junction,27 and thus the band bending formed at the n-Si/CoOx interface could create a strong inversion layer at the surface of n-Si,8,30 which thereby generates a photovoltage for the effective separation, collection and diffusion of charge carriers.27,31 In a sharp comparison to the polished pure n-Si, over 6 h PEC operation in 1.0 M NaOH solution, the n-Si/CoOx photoanode shows no decay in photocurrent density (Figure S2b), implying that the compact CoOx layer could effectively protect n-Si substrate from surface corrosion.

Interestingly, the n-Si/CoOx/NiCuOx photoanode fabricated via the two-step process undergoes an obvious electrochemical activation process during the cycled CV scans, as shown in Figure S3a. During the activation process, both Jph and Js gradually increase with the continuous CV scans. Meanwhile, the overpotential (defined as the


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difference between the water oxidation potential at a current density of 10 mA cm-2 and 1.23 V vs. RHE) and the Tafel slope of the p+-Si/CoOx/NiCuOx electrode gradually decrease with the continuous CV scans (Figure S3b,c), indicating the water oxidation reaction kinetics is improved at the electrode/electrolyte interface in the electrochemical activation process. This electrochemical activation process inspired us to explore the real active components and their transformations during the PEC reactions, by investigating the X-ray photoelectron spectroscopy (XPS) spectra of the nSi/CoOx/NiCuOx photoanode before and after the electrochemical activation process. As depicted in Figure 2a, three main peaks (854.1 eV, 855.4 eV, 860.9 eV) for Ni 2p3/2 are observed in the as-prepared photoanode, revealing that the Ni element in the NiCuOx layer mainly exists in the form of NiOx.32-34 While after the electrochemical activation process, only two main peaks (855.8 eV, 861.6 eV) are observed, indicating that NiOx was transformed into the complex Ni(OH)2/NiOOH compound during the activation process.33,35 Moreover, as illustrated in Raman spectra (Figure S4), after the electrochemical activation process, two intense peaks appear in the range of 600–1200 and 3200-3800 cm-1, with Raman shift at 870 and 3570 cm-1, respectively, which could


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be assigned to β-Ni(OH)/β-NiOOH.36,37 The XPS and Raman analysis together evidences the phase transformation from NiOx to β-Ni(OH)/β-NiOOH at the NiCuOx/electrolyte interface during the activation process. One should also notice that a similar change happens for the Cu 2p3/2 spectra (Figure 2b), with CuOx in the NiCuOx layer undergoing the transformation from CuOx (932.7 eV, 934.1 eV) to Cu(OH)2 (935.1 eV).38,39 These observations illustrate that an active NiCu(OH)x/NiOOH thin layer would form at the electrode/electrolyte interface during the electrochemical activation process, which serves as a more effective OER catalyst, leading to the accelerated water oxidation reaction kinetics. The O 1s XPS spectra were further monitored to evidence the phase conversion of Ni and Cu species during activation process. Three peaks are observed from the O 1s XPS spectra (Figure 2c): the lattice oxygen (-O) (529.6~529.9 eV), the dissociative adsorbed water (-OH) (531.4~531.8 eV), and the surface-absorbed water

(H2O)

(532~532.5

eV).40

The

phase

transformation

(NiCuOx



NiCu(OH)x/NiOOH) is again verified by the increasing -OH/-O ratio during the electrochemical activation process. The possible transformation of the ALD-deposited CoOx layer was also investigated by XPS analysis. As shown in Figure 2d, the Co 2p


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XPS spectra confirm the different oxidation states of CoO and Co3O4 for the asdeposited CoOx layer,41-43 while after the electrochemical reactions, the Co 2p3/2 peaks locating at 779.5 eV and 780.6 eV should be assigned to Co3O4,42 indicating that the mixed-valence CoOx has been largely converted to Co3O4.43 Thus, it could be deduced that the ALD-deposited CoOx layer (electrochemically converted to Co3O4) could be protective

to

n-Si,

while

the

NiCuOx

layer

(electrochemically

converted

to

NiCu(OH)x/NiOOH) would catalyze water oxidation reaction, for stable and efficient PEC water splitting.


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Figure 2. High-resolution XPS spectra of n-Si/CoOx/NiCuOx photoanode before and after the electrochemical activation process: a) Ni 2p spectra; b) Cu 2p spectra; c) O 1s spectra and d) Co 2p spectra. After the electrochemical activation process, all the n-Si/CoOx/NiCuOx photoanodes exhibit PEC performances superior to n-Si/CoOx. To optimize the catalysis effects of the NiCuOx layer with tuned Ni:Cu molar ratios, the NiCuOx layers with different Ni/Cu ratios (molar ratios of Ni(CH3COO)2/Cu(CH3COO)2 varying from 5:1 to 1:2) was deposited


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onto the as-prepared n-Si/CoOx photoanodes by spin-coating method. As shown in Figure S5a,b, all the n-Si/CoOx/NiCuOx photoanodes exhibit cathodically shifted onset potentials and higher Js relative to the n-Si/CoOx photoanode. With the Ni/Cu ratios decreasing from 5:1 to 2:1, the overpotentials of the n-Si/CoOx/NiCuOx photoanodes, defined as the difference between the onset potential and 1.23 V vs. RHE, decrease from -144 to -188 mV, and the corresponding Jph rapidly increases from 14.6 to 16.6 mA cm-2. Whereas, further decreasing the Ni/Cu ratio from 2:1 to 1:2 would lead to poor PEC water oxidation performance, with overpotential increased to -172 mV and the corresponding Jph decreased to 14.2 mA cm-2. As further confirmed by the calculated applied bias photon-to-current efficiency (ABPE), the n-Si/CoOx/NiCuOx photoanode with Ni:Cu tuned to be 2:1 displays the maximum value reaching 1.42% at 1.15 V vs. RHE, which is much higher than other photoanodes with different Ni/Cu ratios (Figure S5c). These results indicate that the n-Si/CoOx/NiCuOx photoanode could achieve the best PEC performances, with the Ni/Cu ratio optimized to be 2:1. In the following discussion, the n-Si/CoOx/NiCuOx photoanode has Ni/Cu ratio tuned to be 2:1, unless specifically noted.


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As observed from the typical characteristics of current-density vs. potential for PEC water oxidation (Figure 3a), the optimized n-Si/CoOx/NiCuOx photoanode shows the onset potential cathodically shifted from 1.14 V to 1.04 V vs. RHE and Jph greatly increased from 4.9 to 16.6 mA cm-2, in comparison to n-Si/CoOx. However, with only NiOx or CuOx overlayer spin-coated onto the n-Si/CoOx photoanode, the obtained nSi/CoOx/CuOx or n-Si/CoOx/NiOx does not show much improvement in photoanodic water oxidation performances and even a more anodic onset potential observed for the n-Si/CoOx/CuOx photoanode, as relative to the n-Si/CoOx photoanode. One can further observe that the p+-Si/CoOx/NiCuOx electrode needs a bias potential 40 mV smaller than the p+-Si/CoOx electrode for the dark current density reaching 10 mA cm-2 (Figure S6). These comparative results clearly demonstrate that the NiCuOx layer with optimized chemical composition is essential to efficiently catalyze the water oxidation reaction. Moreover, the Js of the n-Si/CoOx/NiCuOx photoanode is increased by 20%, as compared to the n-Si/CoOx photoanode, mainly due to the anti-reflection property of the NiCuOx layer to enhance the light capture capability (Figure S7).


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All the above discussions demonstrate that the n-Si/CoOx/NiCuOx photoanode could provide a PEC activity much superior to the n-Si/CoOx photoanode for water oxidation (Figure 3a,b). As an important metric relevant to the n-Si based photoanode, photovoltage is defined as the potential difference between the n-Si based photoanode and the p+-Si based electrode for photocurrent and dark-current density reaching 10 mA cm-2, respectively.9,10,24-27 One can find that the n-Si/CoOx/NiCuOx photoanode encouragingly produces a substantial photovoltage of ~550 mV, relative to the p+Si/CoOx/NiCuOx electrode (Figure 3b, left panel), while the n-Si/CoOx photoanode shows a photovoltage of only ~445 mV in comparison to the p+-Si/CoOx electrode (Figure S8). This photovoltage of the n-Si/CoOx/NiCuOx photoanode is fairly high for a photoanode without buried p+n homojunction or noble metal catalyst, in consideration of the highest value of 630 mV achieved for the np+-Si/TiO2/Ir photoanode with both buried p+n homojunction and noble metal catalyst16 (Table S1). This implies that the earthabundant metal oxide composites (NiCuOx) are of encouraging potentials to replace the noble metal to efficiently catalyze n-Si based photoanodes for water oxidation reaction.


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Figure 3. The PEC performances of the n-Si/CoOx/NiCuOx photoanode: a) CV curves of n-Si/CoOx photoanodes under 1 sun simulated illumination or in the dark in the electrolyte of 1.0 M NaOH, scan rate = 20 mV s-1; b) CV (left panel) and LSV (right panel, with IR-compensation) curves of n-Si/CoOx/NiCuOx photoanode (the red lines) and n-Si/CoOx photoanode (the blue lines) under 1 sun simulated illumination or in the dark, and p+-Si/CoOx/NiCuOx electrode (the solid pink line) under dark in the electrolyte of 1.0 M NaOH, scan rate = 20 mV s-1; c) The applied bias photon-to-current efficiency (ABPE) of n-Si/CoOx and n-Si/CoOx/NiCuOx photoanodes; d) Chronoamperometric photocurrent density–time (I–t) curve of nSi/CoOx and n-Si/CoOx/NiCuOx photoanodes at 1.70 V vs. RHE in 1.0 M NaOH ACS Paragon Plus Environment

electrolyte under 1 sun illumination.

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Linear sweep voltammetry (LSV) curves with IR-compensation were further recorded to investigate the PEC performances of the obtained n-Si based photoanodes (Figure 3b, right panel). Comparatively, the n-Si/CoOx/NiCuOx photoanode shows a high Jph reaching 28.3 mA cm-2, which is approximately 1.6 times that of the n-Si/CoOx photoanode (17.5 mA cm-2). As calculated from the IR-compensated LSV curves (Figure 3b, right panel), the maximum ABPE of the n-Si/CoOx/NiCuOx photoanode is determined to be 1.42% at 1.15 V vs. RHE (Figure 3c), outperforming that of the nSi/CoOx photoanode (only 0.24% at 1.20 V vs. RHE). This ABPE is higher than most nSi based photoanodes and even comparable to some n-Si based photoanodes with p+n buried junction (Table S1).

Figure 3d shows the chronoamperometric stability for n-Si/CoOx/NiCuOx photoanode under bias controlled at 1.70 V vs. RHE in 1.0 M NaOH under simulated solar illumination. During ~23 h continuous operation, the n-Si/CoOx/NiCuOx photoanode exhibits an admirable stability with current density maintaining at ~28-30 mA cm-2. The current density shows slight decay after long time operation, as caused by the


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adsorption of O2 bubbles on the photoanode surface (Figure S9), which could be recovered in the fresh 1.0 M NaOH electrolyte with adsorbed O2 bubbles removed (Figure 3d, inset).44 Under the same anodic condition, the n-Si/NiCuOx photoanode shows a rapid decrease in photocurrent density during the several CV scans (Figure S10), which indicates that the spin-coated NiCuOx layer with numerous pinholes on the surface (Figure S10, inset) can hardly protect the n-Si photoanode from serious surface oxidation, leading to a poor stability. That is to say, for the n-Si/CoOx/NiCuOx photoanode, along with the spin-coated NiCuOx layer effectively catalyzing water oxidation reaction, the ALD-deposited CoOx layer is essential to stabilize the n-Si wafer in aqueous alkaline electrolytes with surface corrosion inhibited.

Mott-Schottcky

(M-S)

and

electrochemical

impedance

spectroscopy

(EIS)

measurements were conducted to study the interfacial electrochemical property and water oxidation reaction kinetics of the n-Si/CoOx and n-Si/CoOx/NiCuOx photoanodes. It is well recognized that the photocurrent onset potential of PEC photoanode is governed by the flat-band potentials (Efb),45 which can be determined by the x-intercept


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of the linear region of the M-S plots. As shown in the Figure 4a, the Efb of the nSi/CoOx/NiCuOx photoanode (0.18 V vs. RHE) is negatively shifted by ~80 mV, relative to the n-Si/CoOx photoanode (0.26 V vs. RHE), implying that the generated band bending at the formed n-Si/CoOx/NiCuOx cascading interfaces is larger than that at the single n-Si/CoOx interface, which could thereby provide a larger driving force for interfacial charge separation and charge transfer processes. Furthermore, with EIS plots (Figure 4b) described by an equivalent circuit model (Figure 4b, inset, and see fitted parameters in Table S2), the bulk charge transfer resistance (RSC1) of nSi/CoOx/NiCuOx photoanode is measured to be 18.78 Ω cm2, much smaller than that of n-Si/CoOx photoanode (527.1 Ω cm2). This significant decrease in RSC1 means that the NiCuOx overlayer deposited on n-Si/CoOx could further effectively accelerate charge carrier migration from the bulk of n-Si wafer to the CoOx/NiCuOx double layer, ascribed to the high photovoltage produced in the formed n-Si/CoOx/NiCuOx triple-junction with large band bending at the n-Si/CoOx/NiCuOx cascading interfaces as evidenced by the M-S analysis. It could be also noticed that the resistance of interfacial charge transfer from the photoanode to the electrolyte (RSC2) is much smaller for n-Si/CoOx/NiCuOx


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(306.7 Ω cm2, the NiCuOx/electrolyte interface) than n-Si/CoOx (3344 Ω cm2, the CoOx/electrolyte interface), again implying that the NiCuOx/electrolyte interface could more effectively inject photoholes from photoanode into electrolyte to accelerate the water oxidation reaction kinetics.46,47

On the basis of above results, for the n-Si/CoOx/NiCuOx photoanode, the pinhole free CoOx layer could effectively protect n-Si wafer from surface corrosion in aqueous alkaline condition to stabilize the n-Si photoanode for PEC water oxidation. Subsequently, the spin-coated NiCuOx layer could not only reduce the optical reflection of the photoanode due to the rough surface (Figure S10, inset), but also accelerate the water oxidation reaction kinetics by acting as an effective OER catalyst. Furthermore, the n-Si/metal oxides (CoOx/NiCuOx) triple-junction at the formed n-Si/CoOx/NiCuOx cascading interfaces could generate a large band bending with a ~550 mV photovoltage provided to promote the photogenerated charge separation.


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Figure 4. Electrochemical characterizations of n-Si/CoOx and n-Si/CoOx/NiCuOx photoanodes: a) M-S plots of n-Si/CoOx photoanode (blue) and n-Si/CoOx/NiCuOx photoanode (red) in 1.0 M NaOH electrolyte in the dark; b) EIS plots of n-Si/CoOx (blue) and n-Si/CoOx/NiCuOx (red) photoanodes under simulated illumination in 1.0 M NaOH at 1 V vs. RHE. Figure 5 illustrates the energy band diagram elucidating the charge transfer processes in the n-Si/CoOx and n-Si/CoOx/NiCuOx photoanodes. For the n-Si/CoOx photoanode, with the CoOx thin layer deposited on the n-Si wafer, an upward band bending will be formed at the n-Si/CoOx interface, ascribed to the more negative Fermi level (Ef) of n-Si relative to CoOx, which provides a driving force for the photogenerated holes to transfer from the n-Si substrate to the CoOx thin layer, thereby reducing the


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water oxidation overpotential. Meanwhile, the transferred holes will get involved in the water oxidation reaction rather than the self-corrosion of n-Si photoanode, resulting in the excellent stability of the n-Si/CoOx photoanode. As deduced from the M-S analysis, the n-Si/CoOx/NiCuOx photoanode shows more negative Efb, in contrast to the nSi/CoOx photoanode. Additionally, the p+-Si/CoOx/NiCuOx electrode requires less overpotential for water splitting relative to the p+-Si/CoOx electrode. It could be then supposed that the formed cascading interfaces in the n-Si/CoOx/NiCuOx photoanode will increase the upward band bending to further promote the photogenerated charge separation by providing a higher photovoltage. Further EIS analysis demonstrates that the charge transfer resistances in bulk and at the photoanode/electrolyte interface can be significantly reduced for the n-Si/CoOx/NiCuOx photoanode in comparison to the nSi/CoOx photoanode, indicating that the well-designed n-Si/CoOx/NiCuOx cascading interfaces could further promote the charge carrier separation and migration in bulk, and simultaneously the formed NiCuOx/electrolyte interface would kinetically catalyze the efficient water oxidation reaction by replacing the less active CoOx/electrolyte interface in the n-Si/CoOx photoanode. As evidenced by Raman and XPS analysis, the


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accelerated water oxidation kinetics should be attributed to the in-situ electrochemical transformation of NiCuOx into NiCu(OH)x/NiOOH with better OER performance at the NiCuOx/electrolyte interface. Synergistically, the n-Si photoanode is stabilized and catalyzed by the CoOx/NiCuOx double layer for highly efficient and stable PEC water splitting.


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Figure 5. The schematic and energy band diagram of n-Si/CoOx and nSi/CoOx/NiCuOx photoanodes under illumination in 1.0 M NaOH electrolyte. With the driving force formed in the n-Si/CoOx and n-Si/CoOx/NiCuOx photoanodes, photogenerated

holes

and

electrons

will

be

transferred

to

the

photoanode/electrolyte interface and the Pt electrode for water oxidation and reduction reactions, respectively. The photovoltage of the n-Si photoanodes are In summary, a novel n-Si/CoOx/NiCuOx photoanode was successfully fabricated by an the difference potential reaching 10 mA cm-2 between the n-Si photoanodes and ALD and spin-coating two-step method, with excellent PEC performance achieved for the corresponding p+-Si electrodes. water oxidation, attributed to the synergy of the formed n-Si/CoOx/NiCuOx cascading interfaces. Under the simulated solar illumination, the n-Si/CoOx/NiCuOx photoanode exhibits a photocurrent density of 28.3 mA cm-2 at 1.23 V vs. RHE, without any photocurrent decay during about 23 h reaction. The maximum APBE was calculated to be 1.42% at 1.15 V vs. RHE, which stands in the highest ABPEs of the n-Si based photoanodes. It was demonstrated that the pinhole-free CoOx thin layer could protect nSi photoanode against surface corrosion, while the spin-coated NiCuOx overlayer could reduce light reflection to increase the saturation photocurrent density. Further


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electrochemical analysis evidences that, with CoOx and NiCuOx layers successively deposited on n-Si wafer, a large band bending would be formed at the well-designed nSi/CoOx/NiCuOx cascading interfaces to produce a high photovoltage (~550 mV), giving rise to greatly promoted interfacial charge separation for efficient charge transfer from nSi to photoanode surface. Moreover, at the formed NiCuOx/electrolyte interface, the NiCuOx

overlayer

would

be

in-situ

electrochemically

transformed

to

the

NiCu(OH)x/NiOOH active species during PEC reaction, which could serve as an effective water oxidation catalyst and the accelerate the OER kinetics at the photoanode/electrolyte interface. This study demonstrates that earth-abundant transition metal oxides could enable n-Si based photoanodes for excellent PEC water splitting, by introducing the well-designed cascading interfaces with multifunctions for promoting charge carrier separation and transfer as well as catalyzing and stabilizing water oxidation reaction.

ASSOCIATED CONTENT

Supporting Information.


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The Supporting Information is available free of charge. Experimental section; Structure characterizations of n-Si/CoOx and n-Si/CoOx/NiCuOx photoanodes; Cyclic voltammetry and the stable performance of n-Si/CoOx photoanode; Cyclic voltammetry of nSi/CoOx/NiCuOx photoanode and p+-Si/CoOx/NiCuOx electrode in different cycles; Raman spectrum of n-Si/CoOx/NiCuOx photoanode; The PEC performances of nSi/CoOx/NiCuOx

photoanode

with

different

Ni/Cu

ratios;

The

electrochemical

performances of p+-Si/CoOx/NiCuOx and p+-Si/CoOx electrodes; Reflectance spectra of n-Si, n-Si/CoOx and n-Si/CoOx/NiCuOx; Cyclic voltammetry of n-Si/CoOx photoanode and p+-Si/CoOx electrode; The real-time picture of n-Si/CoOx/NiCuOx photoanode for water oxidation; Cyclic voltammetry of n-Si/NiCuOx photoanode; Performance comparison of different n-Si based photoanodes for PEC water oxidation; The fitting data of EIS plots for different n-Si based photoanodes.

AUTHOR INFORMATION

Corresponding Authors E-mail: [email protected]


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Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

The financial support from the National Natural Science Foundation of China (No. 21875183 and 51672210), the National Program for Support of Top-notch Young Professionals, and the “Fundamental Research Funds for the Central Universities”.

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Cascading Interfaces Enable n-Si Photoanodes for Efficient and

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